Youtube has been re-encoding their videos into the Webm format and I've encountered my first today and the playback was Horrible on both my Mac Pro tower running fully up to date Snow Leopard, and my new MacBook Pro running up to date Lion playing back in Safari.
Here is the video:
http://www.youtube.com/watch?v=B4foi6nXvDo
Here is the video:
http://www.youtube.com/watch?v=B4foi6nXvDo